The field of the disclosure relates generally to passive wireless sensors and, more particularly, to passive wireless sensors for rotary machines.
A helicopter is a rotary-wing aircraft that uses a system of power-driven rotor blades to generate lift and thrust that enables the helicopter to climb/descend and to move in a horizontal direction. The rotor blades are coupled to, and extend radially outward from, a rotatable shaft, i.e., a rotor shaft coupled to a rotary drive device. A pitch of each blade may be individually adjustable to vary the angle of attack and an amount of lift generated by each blade. In some known helicopters, a pilot is provided with a collective pitch control, which changes the pitch on all of the blades simultaneously and equally, and a cyclic pitch control, which constantly varies the pitch of the individual blades through each cycle of rotation. Moreover, varying the pitch of the rotating blades with the cyclic pitch control varies the torque and strain in the rotary shaft.
The rotational forces induced on the rotor shaft by the rotary drive device, coupled with the forces induced on the rotor shaft due to the blade pitch changes, are monitored to facilitate maintaining such induced forces under predetermined parameters. As such, many of these known helicopter rotor shafts include known sensing devices that are configured to withstand high rotational speeds, e.g., in excess of 25,000 revolutions per minute (rpm) and the stresses and strains associated with such high-velocity rotational effects for only a short period of time, i.e., 100 hours or less. Therefore, stress monitoring devices for such rotary shafts are required to be sturdily constructed and mounted to reduce a potential for in-flight failures and to extend their operational service life. For example, some of such known stress monitoring devices include hardware, such as ground planes and terminating resistors, that substantially increase the construction and maintenance costs of helicopters.